1. Field of the Invention
This invention relates generally to cable modem drivers, and more particularly to a low EMI shutdown driver circuit and method exhibiting very predictable and gentle cable modem start-up behavior.
2. Description of the Prior Art
Currently available technology relating to the present invention includes a Multimedia Cable Networking Standards (MCNS) compliant xe2x80x9cexternal RF switchxe2x80x9d architecture. By way of background, the Institute of Electronic and Electrical Engineering""s (IEEE) 802.14 Cable TV Media Access Control and Physical Protocol Working Group was formed in 1994 to develop international standards for data communications over cable. The goal was to submit a cable modem Media Access Control and Physical Protocol standard to the IEEE in December 1995, but the delivery date slipped to late 1997.
Because of the delay in finalizing the IEEE 802.14 standard, certain cable operators, operating under a limited partnership, dubbed Multimedia Cable Network System Partners Ltd. (MCNS), proceeded to research and publish their own set of interface specifications for high-speed cable data services. MCNS released its Data Over Cable System Interface Specification (DOCSIS) for cable modem products to vendors in March 1997. Many vendors have announced plans to build products based on the MCNS DOCSIS standard.
There is a stringent requirement in the cable modem standard, DOCSIS, for cable modem drivers to transition between the power-up and power-down modes while keeping the disturbance on the line within a very tight limit. Existing solutions suppress the line glitch by using an external RF switch.
In view of the foregoing, a need exists for a bias current generator that exhibits very predictable and gentle start-up behavior and that mitigates the need for an external RF switch such as used with known solutions for suppressing line glitches.
To meet the above and other objectives, the present invention provides a bias current generator that exhibits very predictable and gentle start-up behavior. According to one embodiment, a TANH bias current generator has a xe2x80x98distributedxe2x80x99 output stage comprising a differential pair Q1/Q2 in combination with a pair of resistors R1 and R2 that act as a potentiometer controlling the amount of voltage seen across the input of the differential pair. The distributed output stage has a xe2x80x98steeringxe2x80x99 input that controls the amount of bias current flowing through each transistor (Q1 and Q2). For a given bandgap voltage, ramp voltage and slope, resistors R1 and R2 control the speed of transfer of bias current from transistor Q1 to transistor Q2. For a linear voltage ramp, the voltage across base resistor R1 will be approximately linear, and therefore the current transfer from Q1 to Q2 will take the form of a hyperbolic tangent that will allow a very gentle start-up.
According to another embodiment, a third transistor Q1xe2x80x2 is added to provide a non-zero xe2x80x9cpower-downxe2x80x9d current. This embodiment is used when the bias current is required to be moved between two non-zero amounts. Transistor Q1 has an emitter area xe2x80x9cAxe2x80x9d and transistor Q1xe2x80x2 has an emitter area xe2x80x9cBxe2x80x9d. The ratio of xe2x80x9cpower-upxe2x80x9d to xe2x80x9cpower-downxe2x80x9d current is set by emitter area and is equal to B/A.
In one aspect of the invention, a bias current generator is implemented that exhibits very predictable and gentle start-up behavior.
In another aspect of the invention, a bias current generator is implemented that accommodates moving a bias current between two non-zero amounts.
In yet another aspect of the invention, a bias current generator is implemented that has low EMI during enable and disable modes of operation.
In still another aspect of the invention, a bias current generator is implemented for use in cable modem drivers that are required to meet DOCSIS for cable modem products.
In still another aspect of the invention, a bias current generator is implemented for use with any circuits that are required to transition between low-power and high-power modes in a manner consistent with low EMI.